Agitated Contactors Research Articles

Drop Size Distribution in Agitated Contactor: A Review

The breakage rate of liquid drops in the dispersed phase is a key way to improve the heat and mass transfer between the continuous/dispersed phases. This work includes a review of experimental results of liquid drop breakage in an agitated tank. The study highlighted the experimental conditions that were investigated as well as the important findings about the impact of operating conditions on some breakup parameters. The conflicts and discrepancies in the findings of those studies were identified and analyzed. The review found that many experimental parameters affect the drop breakage rate. The breakage probability (BP), number of fragments, and breakage time (BT) are direct functions of power input.

Continuous preparation of nimesulide nanoparticles by liquid antisolvent precipitation using spinning disc reactor

AbstractBACKGROUNDNimesulide (NIM), a nonsteroidal anti‐inflammatory drug (NSAID) acts as an antipyretic, analgesic and anti‐inflammatory agent, and it is widely used in treating acute pain due to various diseases such as headache, migraine, back pain and postoperative pain. Nimesulide is reported to have poor solubility in water (0.01 mg mL−1). In this work, the solvent–antisolvent (S:AS) precipitation of NIM using spinning disc reactor (SDR) was investigated.RESULTSComparative studies were investigated between stirred tank reactor and SDR. The effects of different functional parameters of spinning disc process such as S:AS ratio and flow rate, nimesulide concentration, disc surface texture, capping agent concentration, different capping agents and speed of rotating disc were studied on the particle size of nimesulide. The average particle size of 192 nm for nimesulide was successfully achieved with a considerable particle size distribution at a 300 mL min−1 total flow rate, with the following parameters: concentration of nimesulide in ethanol 0.5 mg mL−1; concentration of polyvinylpyrrolidone (PVP) in deionized water 1 mg mL−1; S:AS ratio 1:6 and at 3000 rpm rotating disc speed. This result should be compared with 9.5 µm particle size with mechanical agitated contactor at S:AS ratio 1:6 and speed of agitation 600 rpm.CONCLUSIONThe NIM nanoparticles can be continuously prepared in a SDR with a control and uniform particle size. This leads to increased dissolution rate and bioavailability of poorly water soluble drugs. © 2018 Society of Chemical Industry

Bifurcation analysis and multiplicity of steady states in a multistage agitated contactor for gas‐liquid processes

In this work, a bifurcation analysis of steady states in a multistage agitated contactor (MAC) was conducted for typical gas‐liquid processes. A cascade of stirred tank reactors with a backflow was used to model the reactive flow in the MAC, and second‐order reaction kinetics was employed to represent typical gas‐liquid processes. In the developed model, the effect of temperature on gas solubility was taken into account. The multiplicity behaviour of steady states in such a reactor was investigated using a continuation method. Nonlinearity behaviours such as limit points and Hopf points were detected under various process parameters investigated in this work. A stability analysis was also conducted to determine whether the multiple steady state solutions observed were stable. The results suggested that a medium adiabatic temperature rise and small temperature difference are preferred to avoid system instability in operating such reactors.

Process intensification approach for preparation of curcumin nanoparticles via solvent–nonsolvent nanoprecipitation using spinning disc reactor

Continuous preparation of curcumin nanoparticles via solvent–nonsolvent (S-NS) precipitation by using spinning disc reactor was investigated. The process intensification by spinning disc reactor (SDR) along with the comparative study of conventional mechanical agitated contactor was carried out. Solvent used for curcumin precipitation in this study was ethanol whereas non-solvent deionised water. Influences of various operating parameters for spinning disc process; such as flow rate of S-NS, S-NS ratio, concentration of curcumin, disc characteristics, concentration of protecting agent and rotating disc speed were examined on the nanoparticles size. The average optimum curcumin particles size was obtained in the range 180–220nm in consideration with particles size distribution at a flow rate of 200mLmin−1; curcumin concentration of 0.5gL−1 in ethanol; polyvinylpyrilodine (PVP) concentration of 1gL−1 in deionised water; S:NS ratio 1:4 and operating disc speed of 1500rpm. Particles were characterized by using XRD, FT-IR, DSC and SEM which showed decrease in the crystallinity after the nanoprecipitation of curcumin. The dissolution rates of the fabricated curcumin nanoparticle were found drastically higher than original curcumin.

Hydrogenation of nitrile butadiene rubber in a multistage agitated contactor: Experiments and numerical simulation

The performance of a multistage agitated contactor (MAC) with cocurrent reactive gas liquid upflow was studied for the hydrogenation of nitrile butadiene rubber using a homogenous osmium complex catalyst. The hydrogenation performance was investigated under various catalyst concentrations, liquid flow rates, pressures, and polymer concentrations. The desirable hydrogenation conversion of over 95% was achieved under the industrially relevant operation conditions. A comprehensive reactor model consisting of stirred tanks in series with back flow (CTB) was applied to describe the liquid flow behaviour in the presence of the hydrogenation process. The established model satisfactorily predicted dynamic hydrogenation performance considering the gas liquid mass transfer effect and temperature profile in each stage. A modified CSTRs-in-series model accounting for mass transfer effect was proposed to predict the hydrogenation degree at steady state and a good agreement was demonstrated between the model prediction and the experimental data.

Residence Time Distribution in a Multistage Agitated Contactor with Newtonian Fluids: CFD Prediction and Experimental Validation

Residence time distributions (RTDs) are investigated in a multistage agitated contactor (MAC) for Newtonian fluids. A suitable computational fluid dynamics (CFD) simulation strategy is presented in this work. The effects of operating conditions, reactor geometries, and a turbulent parameter for the RTD curves are discussed. The resulting mean residence times and variances via the CFD simulation are in good agreement with the experimental data. Various liquids are used to simulate the RTD under different flow conditions. The Reynolds number with respect to the impeller speed ranged from 2600 to 50 000. The simulation results show that a cascade of stirred tanks with a back flow (CTB) model is more suitable to describe the flow behavior in a MAC with few stages than an axial dispersion model.

Impact of floating suspended solids on the homogenisation of the liquid phase in dual-impeller agitated vessel

The study relates to the liquid phase mixing in dual-impeller agitated contactors of internal diameter 0.32 m in presence of floating solid particles. High-density polyethylene particles and tap water were used as the solid and liquid phase in all experiments. Mechanically agitated tank was provided with two down-pumping pitched blade turbines (PTD). The mixing time has been measured by conductivity measurements using a pulse technique, whereas the minimum impeller speed for the complete drawdown of floating solid particles, N JS, was determined using Joosten visual method. The effects of a number of variables such as floating solids concentrations, the impeller diameter, the off-bottom impeller clearance as well as the spacing between impellers on the critical impeller speed, mixing time and power consumption values were studied in detail. Previous investigations on this subject have been limited to single-impeller configuration only. The dual-impeller reactor behaviour has been found to be very complex and their design must be considered with a great care. Obtained results show that the presence of floating solids in the liquid significantly affects the mixing time of the liquid phase. The analysed parameters are a strong function of the flow patterns structure created by dual-impeller configuration applied. Using experimental data dimensionless correlation is proposed to predict mixing time beyond complete suspension regime.

Optimization‐Based Nonlinear Centralized Controller Tuning of Liquid‐Liquid Extraction Processes

The control problem of an agitated contactor is considered in this work. A Scheibel extraction column is modeled using the non‐equilibrium backflow mixing cell model. Model dynamic analysis shows that this process is highly nonlinear, thus the control problem solution of such a system needs to tackle the process nonlinearity efficiently. The control problem of this process is solved by developing a multivariable nonlinear control system implemented in MATLAB™. In this control methodology, a new controller tuning method is adopted, in which the time‐domain control parameter‐tuning problem is solved as a constrained optimization problem. A MIMO (multi‐input multi‐output) PI controller structure is used in this strategy. The centralized controller uses a 2×2 transfer function and accounts for loops interaction. The controller parameters are tuned using an optimization‐based algorithm with constraints imposed on the process variables reference trajectories. Incremental tuning procedure is performed until the extractor output variables transient response satisfies a preset uncertainty which bounds around the reference trajectory. A decentralized model‐based IMC (internal model control) control strategy is compared with the newly developed centralized MIMO PI control one. Stability and robustness tests are applied to the two algorithms. The performance of the MIMO PI controller is found to be superior to that of the conventional IMC controller in terms of stability, robustness, loops interaction handling, and step‐change tracking characteristics.

Modeling and Simulation of a Multistage Agitated Contactor for Hydrogenation of Nitrile Butadiene Rubber

The dynamic behavior of a multistage agitated contactor has been numerically studied for its potential use for catalytic hydrogenation of nitrile butadiene rubber. A model consisting of stirred tanks in series with back flow is applied to describe the flow behavior in the presence of hydrogenation within a multistage agitated contactor of six stages. The coupled behavior between kinetics and mass transfer is analyzed by manipulating parameters involved in the model. The model predicted that the performance of the reactor is completely governed by the ability of the catalyst in activating hydrogen, the loading level of C=C double bonds, the relative capacity of the reaction to mass transfer, and the backmixing. Mass transfer and backmixing are important factors for the hydrogenation process and enhancing mass transfer and minimizing backmixing helps in increasing hydrogenation conversion. The effects of backmixing and the number of stages for the hydrogenation are discussed, and optimal operation conditions are suggested.

Liquid Backmixing and Phase Holdup in a Gas−Liquid Multistage Agitated Contactor

Axial backmixing of the liquid phase in a multistage agitated contactor (MAC) is investigated through residence time distribution (RTD) experiments over a large range of variables such as gas velocity, liquid velocity, stirring speed, and the number of stages using an air−water system. The experimental RTD curves are satisfactorily compared with the cascade of stirred tanks with backflow (CTB) model. The backflow ratio of the liquid phase is evaluated. The effects of the operating variables on the backflow ratio are examined. The phase holdup in a MAC is also investigated. Based on the data from the present study and the literature, a correlation is developed to predict the backflow ratio in a MAC for an air−water system operated in co-current manner. It is also observed that low backmixing is achieved in the MAC.

Adsorption of an ionizable drug onto microspheres: experimental and modeling studies

The purpose of this work was to study the in vitro equilibria and the adsorption kinetics of an ionizable drug, indomethacin, onto commercially available cationic polymeric microspheres: DEAE Trisacryl LS and QA Trisacryl LS. Isotherms were fitted to theoretical equations allowing accurate predictions of drug loading at different salt concentrations. Isotherm measurements were quickly obtained by simple column breakthrough experiments. The nature of the ion exchange group of the microspheres was observed to be preponderant for adsorption, as the tertiary amine derivative exhibited 53% more capacity than its quaternary amine counterpart. The maximum equilibrium uptake capacity in a 5 mM Tris–HCl buffer at pH 7.4 is 303 mmol/ml of particle volume, for DEAE microspheres. Transport properties of indomethacin into the tertiary amine microspheres were obtained in agitated contactor. Microbeads loading was completed in a 1–6 min range and was found to be controlled by pore diffusion mechanism. Equilibrium uptake data was fitted to the Langmuir and the mass action law models. Adsorption kinetics were fitted to a pore diffusion model. Good correlation was obtained between the theoretical models and the experimental data. The methodology outlined in this work provided a simple approach of estimating adsorption behavior of drugs onto ion-exchange macroporous microspheres. Although significant indomethacin loading was obtained onto the DEAE microspheres, the rapid rate of diffusion is not compatible with sustained release properties sought for this type of microspheres.

Performance comparison of suspended bed and batch contactor chromatography

In some applications, the purification and recovery of biomolecules is performed via a cascade of batch adsorption and desorption stages using agitated contactors and related filtration devices. Suspended bed chromatography is a recent process-scale innovation that is applicable to these separations. This hybrid technique exploits the benefits of combining batch adsorption in an agitated contactor with elution in an enclosed column system. To some extent, the process is similar to batch contactor chromatography but can be fully contained and significantly quicker. The process has two steps; first the fluid containing the sample is mixed with the adsorbent in a stirred tank. Second, the slurry suspension is transferred directly into a specialized column, such as an IsoPak column. The media with the adsorbed product is formed as a packed bed, whilst the suspension liquid is passed out of the column. The product is then eluted from the packed bed utilizing standard column-chromatography techniques. The performance of the suspended bed and the agitated contactor operations are demonstrated both by full-scale experimental results and process simulations. The purification of ovalbumin from a hen-egg white feedstock by anion-exchange chromatography was used as a case study in order to prove the concept. With the availability of both pump-packed systems and shear-resistant media, suspended bed chromatography is a better alternative for a range of applications than the traditional batch separations using agitated contactors.

Regional Mean Bubble Diameters and Gas Holdup in Agitated Gas-Liquid Contactors

Gas-liquid contacting in mechanically agitated vessels is widely used in the process industry. Bubble size measurements at different flow regions in the vessel provide useful information for the mechanisms of gas dispersion and gas-liquid flow. In this work, bubble size distributions and distributions of Sauter mean bubble diameters at four different regions in air-water and air-NaCl solution systems agitated by a six-blade disk turbine are measured by using the photographic method. The effects of gassing rate, impeller speed and electrolyte presence in the system have been examined.

Regional Mean Bubble Diameters and Gas Holdup in Agitated Gas-Liquid Contactors

Regional Mean Bubble Diameters and Gas Holdup in Agitated Gas-Liquid Contactors

OPTIMUM DESIGN OF STATOR-ROTOR ASSEMBLY IN GAS INDUCING TYPE MECHANICALLY AGITATED REACTORS

The stator-rotor combination was optimised in gas inducing type of agitated contactors. The rate of gas induction and the fractional gas hold-up were measured in a 1 m i.d. vessel. Tap water was used as the liquid phase, and air was used as the gas phase. The impeller speed was varied between 1.5 to 6.5 rev/s. Pitched bladed downflow turbine having six blades was used as impeller. The effect of blade angle and blade width was investigated. The stator was optimized in terms of diffuser width, number of stator vanes and the stator-impeller gap. The role of stator air-ring and stator vanes was also investigated. The effect of errors in the fabrication on the performance of the contactor was studied. Ways have been proposed to overcome the deterioration of performance.

Temperature effects on equilibrium and mass transfer of phenylalanine in cation exchangers

The equilibrium and mass transfer rates of phenylalanine in cation-exchange resins with different nominal degree of crosslinking are determined at temperatures in the range 5–85°C. The equilibrium uptake is found to be relatively insensitive to temperature, particularly at high amino acid loadings on the resin. However, mass transfer rates for conditions where the intraparticle resistance is dominant are found to increase substantially as temperature is increased, with the effect being more pronounced for resins of higher degree of crosslinking. Intraparticle diffusivities are obtained from alkaline desorption experiments, while external film mass transfer coefficients in an agitated contactor are obtained from the rates of amino acid uptake from a dilute solution. Intraparticle diffusivities are strongly dependent on the hydration water of the resin and they increase substantially as temperature is increased.

Characterization of protein adsorption by composite silica-polyacrylamide gel anion exchangers I. Equilibrium and mass transfer in agitated contactors

The uptake of proteins by composite porous silica-polyacrylamide gel ion exchangers known under the trade name HyperD, is investigated. These ion exchangers are found to have an exceptionally high static adsorption capacity, greater than about 200 mg/cm3, and a rapid uptake kinetics. Being mechanically quite strong, they are suitable for chromatography applications at elevated mobile phase flow-rates. The uptake equilibrium of proteins by these ion exchangers is obtained experimentally and is found to follow the mass action law. Thus, reversible adsorption and desorption of proteins is obtained by varying the solution ionic strength. Batch mass transfer rates in an agitated contactor under favorable uptake conditions are also obtained. The uptake kinetics is controlled by the fluid phase mass transfer resistance for low protein concentrations, and by the intraparticle mass transfer resistance at high protein concentrations. In the latter case, the uptake is quite rapid and nearly complete saturation of particles 49–76 μm in diameter can be obtained in 5 to 10 min. The parameters of a model describing the batch uptake kinetics in these ion exchangers are obtained by comparing experimental results with model predictions.

Critical impeller speed for solid suspension in turbine agitated contactors

The present study in solid-liquid contactors, an attempt has been made to calculate the minimum/critical impeller speed required for complete suspension of solids. A new correlation, involving Reynolds number, modified Froude number, along with the agitation characteristics is proposed. The effect of impeller geometry as well as impeller clearance are clearly established for 6-blade (flat) turbine agitated contactors.

Flow regimes, gas hold-up and axial gas mixing in the gas-liquid multi-stage agitated contactor

Experimental data are reported on flow regimes, gas hold-up and axial gas mixing of a gas-liquid Multi-stage Agitated Contactor (MAC), consisting of nine compartments [height, H, over diameter, D = 1; D = 0.09 m) separated by horizontal baffles with an opening of 0.04 m and with one centrally positioned impeller per compartment (12-bladed turbine disk; impeller diameter, d I = 0.03 m). For air-water and the liquid in batch, a homogeneous gas-liquid dispersion is realized with a stirring speed, N, above 15 s −1 and a superficial gas velocity, u G, below 0.12 m s −1. These boundaries are affected unfavourably by either a cocurrent or a countercurrent liquid flow. For nine combinations of three gases (air, helium and dichloro-difluoro methane) and three liquids (water, n-octane and monoethylene glycol) the gas holds-up, ε G , and the axial gas mixing were determined with the liquid in batch. Experimental conditions were varied as follows: u G , 0.01–0.09 m s −1; N, 10–36.7 s −1; liquid viscosity, η L , 0.00041–0.021 Pa s; surface tension, σ, 0.02–0.073 N m −1; liquid density, ϱ L , 684–1113 kg m −3 and gas density, ϱ G , 0.16–5.16 kg m −3. For all experiments, the residence time distribution of the gas phase could be described excellently with a Cascade of ideally mixed Tanks in series with Alternating Backflow model (CTAB model). All 85 data ε G could be correlated with an average relative deviation of 11.0% by an extension of Van Dierendoncks' empirical gas hold-up relation. The relative gas backflow (ratio between gas backflow and net gas flow) data could be correlated by a novel relation with average relative deviations of 14.5, 17.7 and 19.5% for air-water (18 data points), helium- n-octane (19 data points) and air-monoethylene glycol (12 data points) systems, respectively.

CRITICAL IMPELLER SPEED FOR SOLID SUSPENSION IN MULTI-IMPELLER AGITATED CONTACTORS: SOLID-LIQUID SYSTEM

Abstract The critical impeller speed for suspension of solid particles (N js ) has been measured in multi-impeller mechanically agitated contactors of 0.15 and 0.30m id and 1.0 m height. Three types of impellers, i.e. disk turbine (DT), pitched turbine downflow(PTD) and pitched turbine upflow(PTU) were used. The number of impellers used in the 0.3 m and 0.15 m id reactors were three and four, respectively. The distance maintained between two impellers was equivalent lo the tank diameter. The effect of impeller type and diameter, particle size and loading, and clearance of the bottom impeller from the reactor bottom was studied and results compared with those of single impeller agitated contactors. PTD impeller was found to be more efficient for solid suspension. The N js values obtained in reactors with multiple impeller are essentially the same as those observed in single impeller reactor and the bottom impeller plays dominant role in determining the N js , values. An empirical correlation has been proposed for estimation of N js and an attempt has been made to explain the mechanism of suspension in multi-impeller agitated reactor.